6 Answers
6

The actual command, including its formal definition posted by @JoshLee is contained in the LaTeX 2e source, section 61.2 Sectioning (p 283). It forms part of ltsect.dtx, the bundle containing all sectioning commands for LaTeX. It even includes a pseudo-code interpretation of the actual macro. Here is an extract:

The
\@startsection{<name>}{<level>}{<indent>}{<beforeskip>}{<afterskip>}{<style>}*[<altheading>]{<heading>}
command is the mother of all the user level sectioning commands. The
part after the *, including the * is optional.

beforeskip: Absolute value = skip to leave above the heading. If negative, then paragraph indent of text following heading is
suppressed.

afterskip: If positive, then skip to leave below heading, else negative of skip to leave to right of run-in heading.

style: Commands to set style. Since June 1996 release the last command in this argument may be a command such as \MakeUppercase or
\fbox that takes an argument. The section heading will be supplied
as the argument to this command. So setting #6 to, say,
\bfseries\MakeUppercase would produce bold, uppercase headings.

If * is missing, then increment the counter. If it is present, then
there should be no [<altheading>] argument. The command uses the
counter secnumdepth. It contains a pointer to the highest section
level that is to be numbered.

It’s not a LaTeX command per se (which is why it’s not well documented), but a TeX macro that’s defined in the implementation of LaTeX’s kernel. @Concerned has a good answer, but you can also explore macros in LaTeX’s interactive mode: